204 CHEMICAL ENGINEERING many instances the coarse rock is received by a large diameter ball mill and reduced in one stage to 50 mesh or finer. Operation of mills in this way practically necessitates the use of a classifier or screen directly following the mill in which the material already sufficient ground may be removed from the pulp, and the unground material returned to the ball mill for further grinding. The capacity of such mills is quite large, but since ball and tube mills are usually considered as fine-reduction machines rather than crushers, this too will be considered under the head of grinding. In addition to the ones already mentioned, there are a large number of machines that have been designed to accomplish this particular work. Some of them have been quite successful, and others not so much so. It would be impossible to consider the merits or demerits of each or any of them in a discussion of this kind, and consequently they are left for study in the light of the particular circumstances under which their use is contemplated. It may not be out of place, however, to consider here some of the principles upon which these machines are constructed. Some crushing machines use in connection with compression a grinding or abrading motion. For example, crushers of the jaw type are in use in which the swinging jaw has, in addition to its swinging motion, a rolling action. These machines are advantageous in that they assist in the discharge of the material crushed. They should be used with discrimination, however, since with highly abrasive rock the wearing plates of such machines are subject to rapid deterioration. They are advisable for use with rock which is not abrasive, such as limestones, shales, clays, etc., which may be expected to have no cutting action. With quartz, granite or the various trap rocks and other hard, siliceous, crystalline aggregates they should not be used. In general, any crushing machine which takes all of the crushing strain directly upon the bearing is not a long lived one. The high stresses developed momentarily and intermittently by the crushing of hard rock are very severe and, mechanically speaking, should not be delivered directly upon a bearing, which ordinarily cannot withstand such heavy work for any great length of time. The best-known types of machines are so constructed that the principal strains are not directly on the bearings, and these types should be used, particularly where very hard rocks are to be operated upon. With soft rocks which do not involve high-breaking strains, machines of this type may be used, but conditions should be carefully studied before installing them for any particular work. Grinding.—The third stage in rock reduction, which we may call grinding, takes a feed from the secondary crushing machines and makes product which will vary according to the subsequent requirements. The desired product may be a granular aggregate varying in size from 10 to 200 mesh, or it may be an entirely — 200-mesh material. For metallurgical purposes, such as cyanide treatment, flotation, etc., a very fine product is often desirable. For concentration, a granular product is required, while for various industrial purposes, such as glass making, pottery, sand production, etc., the requirements may be widely varied. The machinery used for this stage will include many well-known types, such as grinding pans, edge runners, ring-roll mills, fine-grinding rolls and ball and tube mills. These machines involve one or a combination of two or more of all of the crushing principles that have already been enumerated. Their efficiency varies greatly, and durability or wearing qualities are just as variable. Their application will depend largely on the kind of rock to be handled and the use to which the product is to be put. These machines will be considered here according to the various types mentioned.